The production of amorphous EPR elastomers in a gas phase fluidized bed process above their sticking temperatures is difficult due to agglomeration of the sticky, granular resin bed particles under polymerization conditions.
The term "sticky polymer" is defined as a polymer which, although particulate at temperatures below the sticking temperature, agglomerates at temperatures at or above the sticking temperature. The term "sticking temperature", which, in the context of this specification, concerns the sticking temperature of particles of polymer in a fluidized bed, is defined as the temperature at which fluidization ceases due to the agglomeration of particles in the bed. The agglomeration may be spontaneous or occur on short periods of settling.
A polymer may be inherently sticky due to its chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers are also referred to as non-free flowing polymers because of their tendency to compact into aggregates of much larger size than the original particles and not flow out of the relatively small openings in the bottom of product discharge tanks or purge bins. Polymers of this type show acceptable fluidity in a gas phase fluidized bed reactor; however, once motion ceases, the additional mechanical force provided by the fluidizing gas passing through the distributor plate is insufficient to break up the aggregates which form and the bed will not refluidize. These polymers are classified as those, which have a minimum bin opening for free flow at zero storage time of up to two feet and a minimum bin opening for free flow at storage times of greater than five minutes of 4 to 8 feet or more.
Because of the tendency to agglomerate, sticky polymers are difficult to produce in typical gas phase processes, which are usually carried out in fluidized beds. Both economic and safety/environmental considerations indicate, however, that fluidized bed type polymerization is preferred for the manufacture of polymers that can exist in a granular, fluidizable form.
Although polymers that are sticky can be produced in non-gas phase processes, there are certain difficulties associated with the production of such products in, for example, slurry or bulk monomer polymerization processes. In such processes, the diluent or solvent is present in the resins exiting the reaction system at a high concentration leading to severe resin purging problems, particularly if the material in question is a low molecular weight resin or a very low crystallinity resin. Environmental considerations are such that the dissolved monomers and diluent must be removed from the polymer prior to its exposure to air. Safety also dictates the removal of residual hydrocarbons so that closed containers containing the polymers will not exceed safe volatiles levels in the gas head space over the resin. The safety and environmental concerns are accompanied by a definite economic factor in determining a preference for a gas phase fluid bed reaction system. The low number of moving parts and the relative lack of complexity in a basic fluidized bed process enhances the operability of the process and typically results in lower costs of production. Low costs of production are due, in part, to low volumes of recycled process streams and a high unit throughput.
Three major process types have been used for the production of some or all of these sticky resins, i.e., the bulk monomer slurry process; the diluent slurry process; and the solution process. All of these processes, although suitable for the production of many different types of polymers have deficiencies that are not present in the fluidized bed reaction system. The absence of large volumes of solvent or liquid monomer increases the safety of the system. The granular nature of the resultant polymer increases the flexibility of the system in that both granular resin and compounded resin can be delivered to the customer. The granular, porous nature of the polymer also facilitates purging of unwanted monomer to environmentally safe levels. A wide range of molecular weights can be produced in a fluidized bed, i.e., from ultrahigh molecular weights having a melt index of less than 0.001 gram per 10 minutes to relatively low molecular weights having a melt index of up to 100 grams per 10 minutes. Melt index is measured under ASTM Do1238, Condition E, at 190.degree. C., and reported as grams per 10 minutes. The high heat removal capacity of a fluidized bed (due to the circulation of the fluidizing gas) and the ability to control reaction concentrations without the limitations imposed by the solubility of components such as hydrogen in the diluent are also desirable features of the fluidized bed process.
It is clear, then, that the production of polymer by means of a fluidized bed reaction system is advantageous. A typical system of this type is described in U.S. Pat. No. 4,482,687. Unfortunately, this system requires that the granular product be free-flowing. Industry has generally dealt with the problem of sticky polymers by avoiding operating regimes at or above the sticking temperature of the polymer. Low pressure polymerization of olefins in a gas phase reactor using transition metal catalysts is generally performed at temperatures below 120.degree. C.. Where high levels of comonomers are used in combination with ethylene and crystallinity levels are reduced below 30 percent by weight, the sticking temperature of the olefin polymer can be close to the polymerization temperature. Under such conditions, in either a fluidized bed or a stirred gas/solid phase reactor, stickiness of the resin particles becomes a problem. The stickiness problem becomes even more critical with copolymers of ethylene and propylene (EPMs) and ethylene/propylene/diene/terpolymers (EPDMs) having a crystalline content of less than about 10 percent by weight. These particular polymers are also known as EPRs, i.e., ethylene/propylene copolymer rubbers. Commercially desirable EPMs and EPDMs contain about 20 to about 55 percent by weight propylene and the EPDMs contain about 2 to about 15 percent by weight ethylidene norbornene (ENB).
EPRs are practically amorphous with glass transition temperatures of minus 50.degree. C. to minus 60.degree. C. At temperatures above the glass transition temperature, EPM and EPDM are rubbers whose viscosity decreases, like all rubbers, exponentially with increases in temperature. This viscosity decrease with rising temperatures is a major obstacle in the fluidized bed production of EPR because agglomeration increases as particle surface viscosity decreases.
At temperatures above about 30.degree. C., amorphous EPM particles become so sticky that fluidized bed polymerization cannot be carried out reliably. EPDM particles are even stickier than EPM due to the presence of soluble liquid dienes such as ethylidene norbornene.
The stickiness problem can be reduced in a fluidized bed by the introduction of a fluidization aid, and this is described in U.S. Pat. No. 4,994,534. While this procedure is generally effective, it is deficient in the preparation of amorphous or nearly amorphous resins at temperatures at or above their sticking temperatures, i.e., under conditions of maximum stickiness. In this ease, large quantities of the fluidization aid, about 15 to about 50 percent by weight based on the weight of the final product, are required. This, in turn, increases the cost of material; requires large quantities of fluidization aid to be treated to ensure inertness; reduces the polymer throughput of the reactor; increases residues; limits the end use applications of the resin; can affect polymer properties in an undesirable way, e.g., by increasing block or gel formation; and imposes various other economic penalties.
The problem lies, then, in how to produce essentially amorphous or nearly amorphous EPRs at temperatures at or higher than their sticking temperatures, since the higher the temperature the greater the productivity, while at the same time reducing the amount of fluidization aid and, thus, fluidization aid residues, or eliminating the fluidization aid altogether.